The present invention relates to methods for designing lost circulation materials (“LCM”) for use in drilling wellbores penetrating subterranean formations.
Lost circulation is one of the larger contributors to non-productive time during drilling operations. Lost circulation arises from drilling fluid leaking into the formation via undesired flow paths, e.g., permeable sections, natural fractures, and induced fractures. Lost circulation treatments may be used to remediate the wellbore by plugging the undesired flow paths before drilling can resume.
Drilling, most of the time, is performed with an overbalance pressure such that the wellbore pressure, which is related to the equivalent circulating density, is maintained within the mud weight window, i.e., the area between the pore pressure (or collapse pressure) and the fracture pressure at a given depth, see FIG. 1. That is, the pressure is maintained high enough to stop subterranean formation fluids from entering the wellbore and low enough to not create or unduly extend fractures surrounding the wellbore. The term “overbalance pressure,” as used herein, refers to the amount of pressure in the wellbore that exceeds the pore pressure. The term “pore pressure,” as used herein, refers to the pressure of fluids in the formation. Overbalance pressure is needed to prevent subterranean formation fluids from entering the wellbore. The term “fracture pressure,” as used herein, refers to the pressure threshold where pressures exerted in excess of this value from the wellbore onto the formation will cause one or more fractures in the subterranean formation. Wider mud weight windows allow for drilling with a reduced risk of lost circulation.
In common subterranean formations, the mud weight window may be wide, e.g., FIG. 1. However, in formations having problematic zones, e.g., depleted zones, high-permeability zones, highly tectonic areas with high in situ stresses, or pressurized shale zones below salt layers, which are often found in formations with a plurality of lithographies, the mud weight window may be narrower and more variable, e.g., FIG. 2. When the overbalance pressure exceeds the fracture pressure, a fracture is expected to be induced in the formation, and lost circulation may occur. One proactive method of reducing the risk of lost circulation is to strengthen or stabilize the wellbore through the use of LCM. One such method involves shutting-in a drilling fluid comprising LCM and then pressurizing the wellbore so as to induce fractures while simultaneously plugging the fractures with the LCM. Typically, the pressurizing is done as a step-function process until a desired pressure is reached or until a point of diminishing returns (i.e., minimal pressure increases at each step-function). This simultaneous fracture-plug method increases the compressive tangential stress in the near-wellbore region of the subterranean formation (described further herein), which translates to an increase in the fracture initiation pressure or fracture reopening pressure (i.e., an increase in the minimum pressure to initiate or reopen a fracture), thereby widening the mud weight window (e.g., FIG. 3).
Expansion of the mud weight window may translate to cost savings because wellbores that are strengthened to a higher degree allow for safely drilling longer sections of a wellbore, which translates to less non-productive time and decreased costs. Further, longer drilled sections enable longer casing sections. Because each subsequent casing section is at a smaller diameter than the previous section, greater wellbore strengthening may ultimately allow for deeper wellbores and the capabilities to access previously untapped resources.
The plug may perform a variety of functions including keeping the induced fractures propped open, preserving the increased circumferential (hoop) stress that was required to open the fractures, isolating the fracture tips from the fluid and pressure of the wellbore, and any combination thereof. FIG. 4 provides an illustration of a plugged fracture and some of the related stresses including the wellbore pressure that is radially exerted from the wellbore and fluid therein onto the subterranean formation, the hoop stress that is a circumferential pressure in the subterranean formation about the wellbore, the fracture pressure that is the pressure the fluid in the fracture exerts on the proximal portion of the subterranean formation, and the formation pressure that the subterranean formation exerts on, for example, the fracture. The hoop stress is illustrated with arrow pointing toward the fracture, i.e., as a compressive tangential stress, which is the state where the wellbore is stabilized. It should be noted that the formation pressure is also a component of the hoop stress. However, the hoop stress may be a tensive tangential stress with arrow pointing away from the fracture, which is a state where fractures are induced.
As the practice of wellbore strengthening, especially in deep water wells, has increased, so have the number of LCM and potential LCM and related methods. Typically, the choice of which LCM to use, at what concentration (or relative concentrations for more than one LCM) is determined by the properties of the LCM in consideration of the downhole conditions. However, as shown in FIG. 4, the systems downhole can be quite complex with a plurality of stresses and a complex structure of fractures (e.g., uneven surfaces and uneven widths). As such, in the field, many wellbores may be inefficiently strengthened, e.g., with a less effective LCM or at less effective concentrations. Further, in-the-field testing of the various LCM increase the time and cost associated with drilling the wellbore.
Accordingly, understanding how plugs comprising different LCM cause the various stresses experienced in a wellbore to change may advantageously allow for the design of LCM that better strengthen the wellbore, thereby minimizing fluid loss and consequently reducing rig downtime and associated costs.